AP Biology · Unit 8 · Topic 8.4
Add one term to the equation from the last topic and the curve bends. That term is what the environment is allowed to say back.
Learning objective 8.4.A Explain how the density of a population affects and is determined by resource availability in the environment.
Identical rmax, identical starting size, identical number of steps. The only difference between these two runs is one factor: (K − N) / K. Switch it on and off and watch what a single multiplication does.
The factor (K − N)/K is just the fraction of the environment that is still unused. Read the last column downward and you will see why the curve is S-shaped rather than simply flattening.
| Population, N | As a share of K | (K − N) / K | Growth rate, dN/dt | What that means |
|---|
A factor is density-dependent if its effect gets stronger as the population gets denser, and density-independent if it does not. That is the whole test, and it is not about whether the factor is biological. Sort them by comparing the two right-hand columns.
| Factor | Type | Effect at low density | Effect at high density | How it works |
|---|
Two populations, each regulated by one kind of factor, over a hundred and twenty steps. One settles. The other does not, and cannot — not because it is weaker, but because of what it is.
Nothing about the organism changes between these four runs — same rmax, same starting population. Only the environment changes. Watch where each one settles, and how long each takes to get there.
| Carrying capacity | Population at the end | As a share of K | Steps to 90% of K |
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Answer these in your own words before you open the answers.
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